• PRASAD K V S R

      Articles written in Journal of Earth System Science

    • Seasonal variability of tropospheric CO$_{2}$ over India based on model simulation, satellite retrieval and in-situ observation

      KRISHNAPRIYA M RABINDRA K NAYAK ALLAHUDEEN SHAIK BHUVANACHANDRA A DADHWAL V K JHA C S SHESHASAI M V R SASMAL S K PRASAD K V S R

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      In this study, investigation of the seasonal cycle of the tropospheric CO$_{2}$ concentration over India was carried out using the GEOS-Chem atmospheric transport model, Greenhouse gas Observation SATellite (GOSAT) retrievals, and in-situ measurements. The model simulation is highly coherent with the satellite and in-situ datasets, and it shows a distinct seasonal cycle of the tropospheric CO$_{2}$ tendency over India with a negative phase (decreasing concentration) during April–August and a positive phase (increasing concentration) during September–March. The model diagnostics were analyzed to estimate budgets of the surface layer CO$_{2}$, up to 650 hPa pressure level, for the two-phases of the seasonal cycle. A mean tendency, equivalent to −0.70 ppmv month$^{-1}$, observed during April–August, which results from the loss of CO$_{2}$, content in the surface layer through horizontal advection (−2.25 ppmv month$^{-1}$) and vertical diffusion (−0.20 ppmv month$^{-1}$), that dominates the gain from vertical advection (1.53 ppmv month$^{-1}$). The negative contribution of horizontal advection in this period comes from the transport of CO$_{2}$ depleted air-parcels over the oceanic region to India by the southwest monsoon winds and the positive contributions of vertical advection comes from upwelling of CO$_{2}$ enriched air-parcels. The mean tendency, equivalent to 1.01 ppmv month$^{-1}$, during September–March results from the gain through vertical advection (0.78 ppmv month$^{-1}$) and horizontal advection (0.37 ppmv month$^{-1}$) and a small contribution of vertical diffusion (−0.15 ppmv month$^{-1}$). In this period, positive contribution of horizontal advection is due to the transport of CO$_{2}$ enriched air-parcels from the southeast Asian region to India by north-east monsoon winds. At the annual scale, CO$_{2}$ content of the surface layer over India has a net gain of 0.75 GtC that comes from 14.31 GtC through vertical advection that exceeds the loss due to horizontal advection (−11.10 GtC) and vertical diffusion processes (−2.46 GtC). This net gain is almost 85% higher than the input of 0.4 GtC through surface fluxes, which composed of 0.61 GtC anthropogenic emission and −0.21 GtC net terrestrial ecosystem exchanges. Additional sensitivity experiment was carried out to elucidate the semi-annual features of the seasonal cycle of CO$_{2}$ for north India, in contrast to the annual characteristics of the seasonal cycle for south India in relation to the GOSAT observation.

      $\bf{Highlights}$

      $\bullet$ Greenhouse gas Observation SATellite (GOSAT) L3B and L4B retrievals and in-situ flux tower measurements were analysed to describe seasonal cycle of tropospheric CO$_{2}$ over India; and GEOS-Chem atmospheric transport model diagnostics were used to examine the causes of the variability.

      $\bullet$ The seasonal cycle over north India is composed of mixed signature of annual and semi-annual frequencies while south India experiences dominance of annual oscillation. However, the surface layer CO$_{2}$ seasonal tendency has a major negative phase during April–August and a positive phase during September–March.

      $\bullet$ The net negative tendency during April–August results from the loss of CO$_{2}$ from the surface layer through horizontal advection and vertical diffusion processes that dominates the gain from vertical advection; while the net positive tendency during September–March results from the gain through vertical advection and horizontal advection and a small negative contribution from vertical diffusion.

      $\bullet$ At annual scale, the surface layer over India experiences net positive gain of CO$_{2}$ concentration, which is 85% more than the net input from the surface fluxes, and is mostly contributed by large-scale transport processes.

      $\bullet$ Sensitivity experiments were carried out to elucidate the semi-annual features of the seasonal cycle of CO$_{2}$ over north India in relation to the GOSAT observation. It turns out that the secondary trough during October–December on the background of net positive tendency during September–March results from the drawdown of CO$_{2}$ by the terrestrial ecosystem uptake.

    • Investigation of rip current processes along Visakhapatnam beaches, east coast of India: A study based on GNSS drifters and dye experiments

      SURISETTY VV ARUN KUMAR SIVAIAH B VENKATESWARLU CH GIREESH B SRIDEVI T K VENKATESWARA RAO PRASAD K V S R RASHMI SHARMA

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      Rip currents are known as one of the most dangerous hazards on sandy beaches worldwide. Indian beaches are least explored to study the variability of rip currents in space and time. The present study is the first attempt made to understand its dynamics by utilizing indigenously developed GNSS (Global Navigation Satellite System) drifters and Rhodamine-B dye experiments. The Rama Krishna (RK) and Rushikonda Beaches of Visakhapatnam are chosen for this study, due to a recorded number of rip current-related drowning cases observed during the past decade. Few experiments were conducted during pre- and post-monsoon seasons of 2018. Drifters work on the Lagrangian principle, where they measure the current velocities along their paths driven by the surface currents. Error analysis of drifter measurements showed that they are capable of resolving surf zone motions very accurately. Strong rip currents were observed in few locations in the study area, where at times current velocities reached ${\sim}$1 ms$^{-1}$. Also, Rhodamine-B dye was released into the rip current prone zones along with the drifters and observed that the dye patches also followed the drifters. From these experiments, it has been observed that the rip currents are relatively strong during the post-monsoon season, which could be due to the change in the beach morphology. Similar experiments with more number of drifters would help in understanding rip current dynamics and would help in reducing rip current drowning in the beaches.

      $\bf{Highlights}$

      $\bullet$ GNSS based drifters have been designed, developed and field-tested to measure rip currents in the surf zone.

      $\bullet$ Post-Processing Kinematic (PPK) resulted in position estimates with centimeter level accuracy.

      $\bullet$ The drifters are capable to resolve the surf zone motions more accurately in the order of greater than 0.02 ms$^{-1}$.

      $\bullet$ Several drifter deployments and Rhodamine-B dye experiments were carried out at RK and Rushikonda Beaches to measure the dangerous rip currents and coastal currents.

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